With the growing use of computers, image projectors which employ liquid crystal technology are becoming a more popular way to display information on a large screen. Head-mounted display devices are also becoming viable options for an increasing variety of applications. Many of these projectors are comprised of an illumination source, one or more liquid crystal panels with multiple pixels to form an image, and optics to either focus the image as a real image on a screen or to collimate the image for viewing as a virtual image display. Resolution and image quality requirements for color displays such as these continue to increase.
In order to provide color without corresponding loss of resolution, a projector may employ subtractive color imaging. Subtractive color is a well established approach in other imaging technologies, notably color photography and color printing. In a subtractive color display, individual LCD panels or other image sources are stacked upon each other and a white light spectrum is incident upon the stack. Portions of the visible spectrum are selectively subtracted at each LCD panel in order to generate the desired color. The portion of the spectrum which is subtracted is either absorbed or reflected back towards the illumination source. The stacked LCD panels are arranged to be coincident with the light beam so that the image projected is a composite of the images formed by the three panels and that corresponding color pixel elements are aligned in the projected image.
The quality of the subtractive color image is a function of many parameters. Color quality can be excellent but is highly dependent upon the specific color producing mechanisms employed. Achievable pixel count is generally determined by the multiplexing ratio at which adequate contrast and gray scale performance can be achieved. Pixel density, for example in lines per inch (lpi), is limited by the type of LCD approach, the multiplexing ratio for a given size display, and the ability to make reliable drive connections to the display panels.
The stacked nature of a subtractive color projector results in special optical and geometric considerations. With most current display technologies, individual layers cannot be made negligibly thin in the way that photographic and printing color layers can. This introduces special system requirements, such as the need to simultaneously focus three separate color planes and to eliminate parallax problems. Maintaining adequate resolution in the projection system is critical to the competitiveness of the subtractive color approach. In addition, compactness and system stability are important as these are areas in which the subtractive color approach offers key advantages over alternative color display methods, especially for such critical form factor applications as head mounted displays. Yet another key area is minimization of the cost of such a display. In order to justify use of three panels instead of one, the fabrication cost of each must be held to a minimum.
The conventional optical approach to viewing or projecting a subtractive color image is the use of relatively directional or collimated light. Directional in this case can be taken to mean that the angle subtended by the illumination is on the order of the angles spanned by the set of rays passing through each of the corresponding color elements for a given full-color picture element, or pixel. In the case of a hardcopy pixel, such as in a photograph or color thermal transfer image, the layer separation is much smaller than the pixel size, and hence diffuse light can be used. In a typical subtractive color display (see for example U.S. Pat. No. 44,917,465), which might be implemented on an overhead projector using Fresnel lenses and three large light valves, the light is sufficiently directional if the depth of field is sufficient to keep all layers in reasonable focus. In that case, a telecentric configuration makes the magnification equal for all layers, or alternately individual light valves of different sizes can be used. The situation becomes more complicated, though, as the light valve gets smaller, and higher performance is desired.
Although the liquid crystal panels are relatively thin, the individual LCD panels are still located at different distances from the screen upon which the image is projected. With these differences in distance, difficulties arise in focusing the three images at the same time upon a single surface. At high resolutions, where the pixel pitch is significantly smaller than the layer separation, the directional or collimated approach becomes troublesome. The small numerical aperture allowed by constraining light rays to pass through all corresponding color elements constrains both the light throughput for practical light sources and the achievable resolution due to the diffraction limiting aperture effect. Hence it becomes impossible to achieve adequate performance over a single depth of field range using the directional or collimated approach, having a detrimental effect upon the quality of the projected image.
One arrangement which overcomes some of these difficulties consists of a stacked dichroic flat mirror assembly positioned in the path of the projected image (U.S. Pat. No. 5,184,234). The layers of the dichroic mirror assembly selectively reflect the red, green, and blue images generated by the individual LCD panels. The individual mirrors of the mirror assembly are spaced so as to correspond to the spacing between LCD panels. The dichroic mirror assembly is selected such that the first dichroic mirror surface reflects light modulated by the liquid crystal panel that is most remote from the mirror assembly, and passes the remainder of the light beam substantially unaffected. The middle dichroic mirror surface is selected to reflect the image generated by the middle liquid crystal display panel and to pass at least the image light which corresponds to the closest LCD panel. The final reflecting surface is a mirror that will reflect all spectral energy, though only the image formed by the last LCD panel should reach this surface. In conjunction with an imaging lens and a screen, this arrangement provides a color projector with equal path lengths between the LCDs and the image surface, so that the three images can simultaneously be focused on the image surface by the optics.
The arrangement described above has many disadvantages for demanding display applications, such as those utilizing compact miniature AMLCD subtractive color light valves. These light valves are being extended to increasingly higher densities of 500-2000 lpi and beyond, with thousands of pixels on a side, and likely incorporating integrated row and column drive electronics. Unless a completely collimated backlighting arrangement is used, the dichroic mirror assembly of the prior art acts as a tilted slab in the divergent display optical path. As light strikes the surface of the assembly it is refracted due to the different medium. Optical aberrations such as astigmatism and coma are introduced. Further, due to the nature of the mirror stack, the severity of these aberrations is strongly wavelength dependent. This can significantly compromise the performance of an otherwise well corrected system, and require additional complexity in the projection optics if a high resolution display is desired or if high light collection efficiency is needed. In certain desirable configurations, the new aberrations introduced by this arrangement can be nearly as limiting to high resolution performance as the path length differences it is designed to compensate for. Further, complexity is added to the arrangement by the need to provide and maintain proper alignment between the mirror assembly and the LCD panels.
Another drawback of the prior art described above is the requirement for lateral offset between the LCD panels. The clear, or "transparent" areas of each panel must thereby be increased to minimize vignetting or change in image quality in the associated non-overlapped regions. This increase in area can impact the image quality and display panel size, and hence may introduce cost, yield, or other considerations. As an example, assuming the geometry described in U.S. Pat. 5,184,234, and three identical LCD matrix panels with a reasonable separation of 1.5 mm between adjacent active layers, the added dimension would be twice the separation (thickness) between the first and last panels, or 6 mm. An array with a smaller effective pixel size could be used rather than physically increasing the device area, however for a deliberately compact and high resolution light valve, this is not desirable. This is especially true in the case of miniature active matrix substrates which might be fabricated using standard IC processes and design rules. When incorporating bus lines, active elements such as thin film transistors (TFT's) and integrated row and column drive electronics on the substrate, this added dimension could represent a very significant size increase, decreasing the number of devices per wafer, decreasing the device yield, increasing the cost, increasing package size, and possibly increasing the cost and complexity of the photolithography system required.
Yet another disadvantage of the projectors of the prior art is the susceptibility to aliasing, or Moire artifacts caused by spatial interference between the pixel grid structure of the individual, stacked imaging devices. In the case of an active matrix LCD, which is generally preferred when maximum performance is required in terms of pixel count, density, grayscale, contrast and response time, the grid structures are typically opaque and can be quite significant. The Moire artifacts are essentially parallax effects similar to the parallax seen between the modulated image layers, except in this case comparable grid structures subtract light of all wavelengths in all layers. This restricts the effectiveness of the prior art to either highly directional illumination or to devices without appreciable opaque grid structures, such as passively multiplexed twisted nematic LCDs, including super-twisted nematics.